Fix expression filter pushdown

[patricklucas]

Feel free to adjust.

Closes #464

[Tishj]

Thanks for the change, this isn't reviewable or mergable without an accompanying test however

[patricklucas]

Added a test (and test table).

On main:

[10/115] (8%): test/sql/local/iceberg_scans/expression_filter.test
1. test/sql/local/iceberg_scans/expression_filter.test:28
================================================================================
Wrong row count in query! (test/sql/local/iceberg_scans/expression_filter.test:28)!
Expected 2 rows, but got 0 rows
================================================================================
SELECT id, CASE WHEN value = 'foo' THEN 'not null' ELSE NULL END AS role FROM my_datalake.default.expression_filter WHERE role IS NULL ORDER BY id ASC;
================================================================================
Expected result:
================================================================================

2	NULL
3	NULL
================================================================================
Actual result:
================================================================================
id	role
BIGINT	VARCHAR
[ Rows: 0]


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unittest is a Catch v2.13.7 host application.
Run with -? for options

-------------------------------------------------------------------------------
test/sql/local/iceberg_scans/expression_filter.test
-------------------------------------------------------------------------------
/Users/plucas/dev/oss/duckdb-iceberg/duckdb/test/sqlite/test_sqllogictest.cpp:247
...............................................................................

test/sql/local/iceberg_scans/expression_filter.test:28: FAILED:
explicitly with message:
  0

Passes on this branch.

[Tishj]

Thanks, looks great!